Study of jet engine plume phenomena on laser beam pointing and tracking. Work Package 2 TA108.019 Laser Beam propagation and imaging through severe environments

Optical systems on aircraft will experience disturbances from the engine efflux when pointed backwards from the aircraft. The high level of turbulence, which is created when the hot exhaust leaving the engine at high speed is mixed with the surrounding air, may severely limit the performance of such systems. This is especially problematic for platform protection systems such as DIRCM (Directed Infra Red Counter Measures) that may need to point in the vicinity of, or through, the engine plume to protect against missile attacks from behind. To study these effects the TA108.019 collaboration has performed a trial with a downscaled (sub-scaled) jet engine test rig at Volvo Aero in Trollhättan. Turbulence effects were measured through study of laser beams passing along and across the engine flow axis. The measurements showed that, depending on engine thrust and distance to engine axis, a laser beam propagating along the engine axis could be deflected by up to 350 µrad (rms). This is more than is expected in the atmosphere over relevant distances and may severely degrade the efficiency of a laser based DIRCM system in these conditions. With a full scale engine the effects are expected to increase and it is probable that jamming of a missile seeker through the center of the engine plume may be limited by taking longer for the jamming to be effective.